1. Field of the Invention
The present invention relates to disk drives and more particularly to a technique for repeatable runout compensation for alternate tracks on a disk of a disk drive to facilitate manufacturing processes.
2. Description of the Prior Art
In order to remain competitive, disk drive manufacturers must continually provide increased storage capacity while rigidly controlling manufacturing cost. One key to increased storage capacity is increasing track pitch. A critical limiting aspect of increased track pitch is the ability of the disk drive servo system to maintain a read/write head location within a budgeted distance from a track centerline to avoid corrupting data on adjacent tracks during a write operation. This distance is sometimes known as xe2x80x9cTrack Misregistrationxe2x80x9d or TMR. One influence on the TMR budget is the effect of written in runout during servowriting. During operation, the disk drive attempts to follow the path of a track on the disk defined by the position of embedded servo sectors laid down during servowriting. The embedded servo sector positions can deviate from a so-called perfect circle due to, for example, vibrations, which occur during the servowriting process. Such vibration causes the track to thenceforth exhibit a track-dependent repeatable runout (RRO) which appears to the servo system as a position error signal (PES) for each servo sector in the track. The error can be xe2x80x9clearnedxe2x80x9d and compensated for over a relatively lengthy process which scales with the number of tracks and the number of iterations for learning.
Despite the apparent advantages of compensating for RRO, present practice in the industry simply factors in a statistical budget for the error and scraps tracks which appear to have deviation beyond what the budget allows. This is driven by the economics of volume manufacturing, which dictates that increased time and capital equipment required would not support the benefit from RRO learning. Nevertheless, the demand for capacity continues to challenge servo system designers as the track pitch of disk drives grows inexorably smaller.
Accordingly, there exists a need for a disk drive having improved RRO compensation without significantly decreasing manufacturing throughput or unnecessarily increasing manufacturing costs.
The present invention may be embodied in a method, and related apparatus, for determining a plurality of RRO compensation value sets for a plurality of tracks on a disk in a disk drive wherein the disk drive is initially manufactured and then shipped for installation in a host system. Each track comprises a plurality of embedded servo sectors that define a circumferential path that may have RRO requiring compensation during track following. The method comprises determining, during the initial manufacturing of the disk drive, first RRO value sets for a first portion of the tracks to a first desired learning level by measuring the RRO for the first portion tracks based on a first predetermined number of disk rotations. The first portion tracks are not adjacent to each other. Then, after the disk drive is shipped for installation in the host system, determining RRO value sets for a second portion of the tracks to a second desired learning level by measuring the RRO for the second portion tracks based on a second predetermined number of disk rotations.
The first and second predetermined number of disk rotations may be at least 20 disk rotations. The first portion tracks may be about 10% to 50% of a total number of the plurality of tracks. Each first portion track may be separated from another first portion track by one or more second portion tracks.
The tracks may be bi-sequentially addressed from an inner-most first portion track to an outer-most first portion track and then from an outer-most second portion track to an inner-most second portion track. Alternatively, the tracks may be bi-sequentially addressed from an outer-most first portion track to an inner-most first portion track and then from an inner-most second portion track to an outer-most second portion track.
Further, after the disk drive is shipped for installation in the host system, second RRO value sets may be determined for the first portion tracks to a third desired learning level by measuring the RRO for the first portion tracks based on at least a third predetermined number of disk rotations. The third predetermined number of disk rotations is greater than the first predetermined number of disk rotations. The first predetermined number of disk rotations may be at least 4 disk rotations and the second and third predetermined numbers of disk rotations may be at least 100 disk rotations.
Additionally, before the step of determining the first RRO value sets for the second portion tracks, the first portion tracks may be interleaved with the second portion tracks and user data stored only on the first portion tracks. After the step of determining the first RRO value sets for the second portion tracks, the user data stored on the first portion tracks may be rewritten and stored on both the first and second portion tracks such that the data is written on adjacent tracks in sequential order.